Anticorpos: Estrutura e FunçãoAntígenos

Reações entre Antígenos e AnticorposAplicações

Profa. Isabel Santosimsantos@fmrp.usp.brr. 153267

A Natureza dos Anticorpos

Pesquisadores injetavam coelhos com diferentes antígenos

proteínas solúveis, bactérias, hemácias heterólogas....

Obtinham:• Precipitinas• Lisinas• Aglutininas• Opsoninas

PerguntasEstruturas diferentes para cada função?

XTeoria Unitária?

Desenvolvimento do Modelo da Imunoglobulina de Quatro Cadeias

Rodney Porter tratou anticorpos com a enzima papaína e obteve três fragmentos, dois dos quais se ligavam a antígenos, cada um contendo um único sítio de ligação, e o outro fragmento se cristalizou

George Edelman tratou anticorpos com agente redutor que quebra pontes de dissulfeto e obteve indícios de que os anticorpos continham ao menos dois tipos de cadeia

Nisonoff tratou tratou anticorpos com a enzima pepsina e obteve um fragmento com dois sítios de ligação com antígenos e vários peptídeos pequenos

Proteolytic fragments of an IgG moleculeUnraveling structure and function

Structure of an antibody molecule.

The hinge region of the immunoglobulin molecule allows flexibility in binding to multiple antigens.

Adiante: Classes e subclasses de Igs

immunoglobulin-like domains: present in many proteins of the immune system (KIRs of NK cells; involved in cell–cell recognition and adhesion. Together with the immunoglobulins and the T-cell receptors, these proteins make up the extensive immunoglobulin superfamily

O sítio de ligação com antígenos

Hypervariable regions in Ig moleculesWu-Kabat Plots

The interaction of the antibody molecule with specific antigen.

The hypervariable regions lie in discrete loops of the folded structure.

Structure of an Ig domain

Binding of an antigen by an antibody

The constant region confers functional specialization on the antibody.

Human Antibody Isotypes

Human Antibody Isotypes

aDepends onallotype. bForA/Aisomer. cMultivalent binding to transfected cells. Adapted from Bruhnsetal.(2). dAssociation constant (X 106 M-1) for monovalent binding (2). eAfter repeated encounters with protein antigens, often allergens.

*

IgG facts and figures

Heavy chains: g 1 g 2 g3 g4 - Gamma 1 - 4

Half-life: IgG1 21 - 24 days IgG2 21 - 24 days

IgG3 7 - 8 days IgG4 21 - 24 days

Serum level (mgml-1): IgG1 5 - 12 IgG2 2 - 6

IgG3 0.5 - 1 IgG4 0.2 - 1

% of Ig in serum: IgG1 45 - 53 IgG2 11 - 15

IgG3 3 - 6 IgG4 1 - 4

Complement activation: IgG1 +++ IgG2 +

IgG3 ++++ IgG4 No

Interactions with cells: All subclasses via IgG receptors on macrophages

and phagocytes

Transplacental transfer: IgG1 ++ IgG2 +

IgG3 ++ IgG4 ++

FcRn contributes to the long half-life of IgG molecules

Flexibility of antibody molecules

Membrane and secreted forms of Ig heavy chains

Idiotype network: a form of immunoregulation?

Antibodies bind to conformational shapes on the surfaces of antigens using a variety of noncovalent forces.

Antigens can bind in pockets, or grooves, or on extended surfaces in the binding sites of antibodies.

The nature of antigenic determinants

The dose of antigen used in an initial immunization affects the primary and the secondary antibody response.

Antibodies can be elicited by small chemical groups called

haptens only when the hapten is linked to an

immunogenic protein carrier.

Valency and avidity of antibody-antigen interactions

Antigen-antibody complexesThe basis of immunopathology of immune-mediated diseases

Changes in antibody structure during humoral immune responses

Ig expression during B lymphocyte maturation

Propriedades das Igs dependem do tipo e nível de glicosilação

Crystal structure of na human IgG1 molecule viewed from two different angles, demonstrating theflexibility of

the two Fab fragments with respect to each other and the Fc tail.

The N-linked glycan found at position 297 can be found as a core structure, common to all IgG found in human

beings and rodents (core structure indicated with a red dashed line)

Propriedades das Igs dependem do tipo e nível de glicosilação

• Type 2 diabetes mellitus is a common complication of obesity• Activation of the IgG receptor FcgRIIB in endothelium by hyposialylated IgG has a role

in obesity-induced insulin resistance• Despite becoming obese on a high-fat diet, mice lacking FcgRIIB globally or selectively

in endothelium were protected from insulin resistance (insulin delivery to skeletal muscle and resulting maintenance of muscle glucose disposal)

• IgG transferred from patients with T2DM but not from metabolically healthy subjects caused insulin resistance in IgG-deficient mice via FcgRIIB

• IgG from T2DM patients was hyposialylated. • In HFD-fed mice, supplementation with a sialic acid precursor restored IgG sialylation

and preserved insulin sensitivity without affecting weight gain.

University of Texas Southwestern Medical Center, Dallas, Texas, USA

N-acetil-D-manosamina (ManNAc) é precursor de ácido siálico

Some species generate antibodies with alternative structures.

Editorial Summary - Milking cows for their antibodiesHIV infection gives rise to the generation of broadly neutralizing antibodies in a subset of infected subjects. However, it has been difficult to induce such antibodies through vaccination in humans and a variety of animal models. Four cows were immunized with a recombinant HIV envelope protein and broadly neutralizing antibodies developed rapidly after repeated immunization. For example, one cow developed cross-neutralizing activity just 42 days after a vaccine boost.

Effector functions of antibodies

Como detectar e medir Anticorpos e suas interações com Antigenos?

Antígenos particulados - Aglutinação

The Ouchterlony reaction: • Detect, identify, and quantify

antibody and antigen• Test the similarity between antigens

• For disease diagnosis

Antígenos solúveis - Precipitação

The Coombs direct and indirect anti-globulin tests for antibody against red blood cell antigens.